In case of parallelism between a.c. transmission lines and metal pipes for, for example, natural gas, the normal operating current of the transmission line induces a voltage in the metal pipe. For example, from a 400 kV line with an operating current of 1000 A at a distance of 50 m from the pipeline, an induced voltage of about 20 V/km can be obtained.
A metal pipe of the above kind may, for example, constitute part of a long gas conduit, which is disposed in the ground and possibly partially also in water. A conduit of this kind is usually divided into sections with the aid of electrically insulating joints. The length of one section may vary from several kilometers up to several tens of kilometers. If a transmission line runs parallel to such a line for a distance of some length, induced voltages of a considerable magnitude may therefore occur.
When the alternating voltage between the pipe and the surrounding ground (water) exceeds a few tens of volts, this may entail an increased risk of corrosion damage to the pipeline because of electrolytic corrosion. Metal pipes of the kind in question are provided with a protective coating of an electrically insulating material. However, damage unavoidably arises in this coating, whereby the metal pipe is brought into electrical contact with the surrounding medium. At these points the above-mentioned risk of corrosion occurs.
Different types of measures for protection against corrosion are previously known. However, these do not provide any protection against the risk of corrosion which is caused by alternating voltages induced in a pipeline.
A previously know device for corrosion protection of a pipeline disposed in the ground or in water is disclosed in Swedish published patent application 466 160. This device aims at protecting the pipeline against corrosion caused by potential differences in the medium surrounding the pipeline (ground potential differences). The device primarily relates to those cases where the ground potential differences are caused by electric currents flowing in the medium, which typically originate from electric d.c. power plants. The difference in ground potential between two points on the pipeline is sensed by means of ground electrodes disposed near the line. A d.c. source is connected to two points on the line and is adapted to feed a current through the line between these points. The current is controlled in such a way in dependence on the sensed ground potential difference that the voltage drop caused by the current along the pipe corresponds to the ground potential difference.
It is also stated that the device can be designed so as to protect against corrosion in those cases where the ground currents consist of alternating currents. The ground potential difference then consists of an alternating voltage, and instead of a d.c. source an a.c. source is arranged to drive a suitable alternating current through the pipeline.
This known device provides corrosion protection for those cases where the risk of corrosion originates from ground potential differences. However, the device provides no protection at all against the risk of corrosion which is caused by voltages induced in the pipeline.